Stigmatic Self-Incompatibility and Mating Patterns in Trillium grandiflorum and Trillium erectum(Melanthiaceae)

Post-pollination processes governing mating patterns in Trillium , a well-known genus of insect-pollinated woodland herbs, are poorly understood. Mechanisms influencing outcrossing were investigated in T. grandiflorum and T. erectum, two widespread species native to eastern North America. In southern Ontario, Canada, the two species are often sympatric; they flower in early May, and are pollinated by different assemblages of insects. Controlled cross- and self-pollinations and structural observations of pollen germination and pollen tube growth were conducted to determine whether the two species possess a self-incompatibility (SI) system and, if so, the specific site(s) of self-rejection. Controlled pollinations indicated that both species set significantly more seeds from cross-pollination than self-pollination, implicating the action of SI. This was confirmed by structural studies which demonstrated that self-recognition and rejection reactions occurred on dry-type stigmatic papillae. Observations of pollen hydration revealed that self-rejection was rapid, being initiated within 10 min of pollination and prior to pollen tube emergence. Final self-rejection resulted in failure of pollen tube growth at the base of stigmatic papillae. SI was expressed more weakly in T. erectum and thereby resulted in considerable self-seed set in some individuals . Estimates of outcrossing rates using allozyme markers indicated that T . erectum displayed a mixed-mating system whereas T . grandiflorum was more highly outcrossed. Structural studies of pollen traits indicated that the two species differed with respect to the size of grains and their aggregation with implications for pollen dispersal and mating. The ecological and evolutionary implications of the variable expression of SI in Trillium are discussed.